The Prime Minister said the reports about possible chemical warfare were “straight out of the Russians’ playbook” in a discussion on Sky News’ Beth Rigby Interviews programme on Thursday.
“The stuff that you’re hearing about chemical weapons is straight out of their playbook,” he told Sky News.
“They start saying that there are chemical weapons that have been stored by their opponents or by the Americans. And so when they themselves deploy chemical weapons, as I fear they may, they have a sort of maskirovka – a fake story – ready to go.
“You’ve seen it in Syria. You saw it in even in the UK. That’s what they’re already doing. It is a cynical, barbaric government.”
Mr Johnson said Mr Putin’s “best bets” were to ceasefire and allow peaceful negotiations to go ahead.
However, the Prime Minister pointed out only the Russian president himself can realise he has made a “catastrophic mistake now… Or a bit later on”.
His comments echoed the warning from White House press secretary Jen Psaki.
She said Russia could be laying the groundwork for the deployment of chemical weapons.
Her tweet was a direct response to Russia’s claim Ukraine were running a chemical and biological weapons lab with the support from the US.
“This is all an obvious ploy by Russia to try to justify its further premeditated, unprovoked, and unjustified attack on Ukraine,” Ms Psaki tweeted.
“Now that Russia has made these false claims, and China has seemingly endorsed this propaganda, we should all be on the lookout for Russia to possibly use chemical or biological weapons in Ukraine, or to create a false flag operation using them.”
Also on Thursday, Foreign Secretary Liz Truss issued a “rallying call” to allies to “be tough, get peace”.
In a speech in the US, the Cabinet minister said the West’s prior “illusions” that “the spread of peace and stability was inevitable” had been “shattered” by Russian president Putin’s aggression towards his neighbour.
“We knew what Putin was doing. We had the intelligence… but actually we didn’t need it, because Putin announced his designs on Ukraine in public,” she said.
“He set his plans down in black and white and put them up on the Kremlin’s website. But still we didn’t want to believe it.
“Well, we believe it now. The world has woken up. The era of complacency is over. We must rise to this moment.”
The Ukraine war entered its 15th day of the war.
The world reacted in horror when Russian rockets attacked a maternity hospital in Mariupol and children were left in the wreckage, Volodymyr Zelenskyy said.
A six-year-old child was among three people killed after the bombing at the hospital, the city’s deputy mayor has said.
Armed forces minister James Heappey condemned the attack as a “war crime” on Thursday morning as rescuers continued a search for survivors trapped underneath the rubble.
The mayor of Kyiv also revealed around two million people have now fled the capital in fear for their lives in the war-torn country.
Meanwhile, the UK government announced sanctions against Chelsea FC owner Roman Abramovich as well as host of other oligarchs.
His assets will be frozen though the club has been granted a special licence to continue football-related activities.